In the viscosity measuring system of a fluid such as liquid, there are capillary viscometers, falling ball viscometers, rotational viscometers, vibratory viscometers, and so on. Among these viscometers, the vibratory viscometers have many advantages such as a continuous measuring ability allowed by the quick response, a wide dynamic range, a capability of the measurement even with a small amount of the fluid, an easy operation, and a capability of the measurement even during the fluid flowing.
Because of above advantages, the vibratory viscometers have been widely used for measuring the viscosity of the fluid. The technical concept disclosed by Patent Document 1 is featured in that a tuning fork is used for cancelling the reaction by the oscillation of the oscillator. Since the oscillator is a flat plate, however, the real part and the imaginary part of the resistance affecting on the oscillator are of the same value. Therefore, a measured value can be obtained only by the product of the viscosity and the density. Thus, in order to obtain the viscosity, it is necessary to know the density in advance or to divide the above measured value by the density that has been separately measured by a density meter.
Patent Document 2 discloses that the characteristic oscillation period is measured by vibrating a measurement cell in which a sample fluid is filled, and that the density of the measured sample is calculated from the known densities of the pure water and the air. In this method, the sample fluid is required to be filled in the measurement cell, which makes the inline measurement difficult.